1. Field of the Invention
The present invention relates to a display device including a transistor and method of driving the same. Specifically, the present invention relates to a display device having a pixel including a thin film transistor (hereinafter, also referred to as a TFT) and method of driving the same.
2. Description of the Related Art
In recent years, a thin display (also called a flat panel display) using an element which emits light with electrooptic property of liquid crystal or electroluminescence has attracted attention and the market thereof is expected to expand. A so-called active matrix display where pixels are formed with TFTs over a glass substrate have been regarded as important as a thin display. In particular, a TFT having a channel portion formed of a polycrystalline silicon film can achieve a high-speed operation since it has high electron field-effect mobility in comparison with a conventional TFT using an amorphous silicon film. Therefore, pixels can be controlled with a driver circuit which is formed by using TFTs over the same substrate as the pixels. A display in which pixels and various functional circuits using TFTs are formed over a glass substrate has various advantages such as reduction in the number of components, improvement in yield by a simplified manufacturing process, and improvement in productivity.
An active matrix display where electroluminescence elements (also referred to as OLED: Organic Light-Emitting Diode and hereinafter also called “EL element” or “light-emitting element” in this specification) and TFTs are combined has attracted attention as a thin and light display and has been actively studied within both domestic and international. Such a display is also called an organic EL display (OELD) and is examined to be developed to be in practical use as displays with various sizes, from a small size of 2 inches to a large size of over 40 inches.
In general, when an EL element deteriorates, current to voltage applied to the EL element flowing in the EL element is reduced. Current flowing in an EL element and luminance of the EL element are in a proportional relation; therefore reduction in current flowing in the EL element leads to reduction in luminance of the EL element. In addition, in an EL element, a voltage-current luminance characteristic deteriorates more than a current-luminance characteristic. For example, luminance of an EL element deteriorates early when fixed voltage is kept applying thereto compared with when fixed current is kept applying thereto. That is, deterioration in an EL element is easily caused when the EL element is driven in voltage compared to when the EL element is driven in current.
As a driving method for an active matrix EL display using an EL element as a display medium and having a structure in which the EL element and a TFT (hereinafter, also referred to as a driving TFT) are connected in series between two power supply lines, the following methods are known: a method in which a driving TFT operates in a saturation region to change voltage between a gate and source of the driving TFT, thereby controlling a current value flowing to the EL element, and a method in which a driving TFT operates in a linear region, thereby controlling time in which the EL element is supplied with voltage and emits light. In addition, in the driving method in which a driving TFT operates in a saturation region, a driving method in which time in which current flows to an EL element in a certain period is controlled, thereby displaying a gray scale is also known.
In the method in which a driving TFT operates in a leaner region, when the driving TFT is on, potentials of two power supply lines are applied almost as they are to an EL element. That is, the EL element is operated by voltage. As described above, luminance of an EL element deteriorates more when the EL element is operated by voltage compared with when the EL element is operated by current. Therefore, even when luminance of an EL element is the same, the luminance deteriorates more when a driving TFT is operated in a linear region compared with when the driving TFT is operated in a saturation region. Therefore, it can be said that burn-in is easily generated in an active matrix EL display in which a driving TFT is operated in a liner region compared with an active matrix EL display in which a driving TFT is operated in a saturation region.
To prevent burn-in in an active matrix EL display in which a driving TFT is operated in a linear region, a method is known in which deterioration conditions in all EL elements are measured and the EL elements are driven by video signals (see Patent Document 1). In this method, current values of EL elements supplied with certain voltage are measured in each pixel. When there is a deteriorated pixel with a low current value, video signals for the deteriorated pixel is corrected so as to obtain a predetermined current value, which means to obtain predetermined luminance.    [Patent Document 1] Japanese Patent Laid-Open No. 2003-195813
However, in a conventional art, a condition in which characteristics of light-emitting elements are detected is important since current flowing in a light-emitting element in each pixel is small (approximately several μA) when a pixel is formed with an EL element, which is a light-emitting element using a light-emitting medium containing an electroluminescence material. For example, if a detecting condition is different, characteristics of one light-emitting element change significantly and effect of a noise, which is an external factor, also changes significantly.